This invention concerns improvements in and relating to synthetic linear polyester filaments, and more particularly to the dyeability, thermal stability and texturability of such filaments, and to processes for the production of such filaments.
Polyester filaments have been prepared commercially for more than 25 years, and are now manufactured in large quantities amounting to billions of pounds annually. Most of this commercial manufacture has been of poly (ethylene terephthalate). These commercial polyester filaments have been difficult to dye, e.g. as mentioned by H. Ludwig in Section 11.4 "Dyeing Properties" of his book "Polyester Fibers, Chemistry and Technology", German Edition 1964 by Akademie-Verlag and English translation 1971 by John Wiley and Sons Limited. Special dyeing techniques have therefore been used commercially, e.g. dye bath additives called "carriers" have been used to dye the homopolymer, usually at higher pressures and temperatures, or the chemical nature of the polyester has been modified to increase the rate of dyeing or to introduce dye-receptive groups, e.g. as discussed in Griffing & Remington U.S. Pat. No. 3,018,272. These special techniques involve considerable expense, and it has long been desired to provide polyester filaments having useful physical properties, e.g. for apparel and home furnishing applications, but having a dyeability more like that of natural fibers, such as cotton, or cellulosic fibers, such as viscose rayon, which can be dyed at the boil within a reasonable period of time without the need for special techniques of the type referred to. Any reduction in the amount of carrier used in desirable for ecologic as well as economic reasons. Although there have been many suggestions for solving this long-standing problem, it has still been necessary, in commercial practice, to use special dyeing techniques or to introduce chemical modification, as indicated above.
For most consumer purposes, polyester filaments should have good thermal stability, i.e. relatively low shrinkage and preferably over a large temperature range. The maximum permissible shrinkage may vary depending on the intended use, but a boil-off shrinkage of less than about 2% in the final fabric has become generally accepted as necessary for consumer applications. Hitherto, commercial polyester yarns have been prepared with considerably higher boil-off shrinkage, e.g. 8 to 10%, so it has been customary to prepare fabrics with these yarns and then reduce the boil-off shrinkage by heat-setting the fabric. Any new polyester yarns should have been a boil-off shrinkage no higher than is customary. It would also be advantageous to be able to prepare continuous filaments having the desired low boil-off shrinkage directly, i.e. by spinning such filaments without any need for further treatment such as heat-setting. A low shrinkage at higher temperatures (120-200.degree. C), such as are usually encountered during textile finishing and pressing operations, i.e. a low dry heat shrinkage, would also be desirable. Hitherto, most commercial polyester filaments have had a dry heat shrinkage significantly more than their boil-off shrinkage. It has long been desired to provide a polyester yarn with good thermal stability when subjected to either boil-off or such dry heat at higher temperatures.
Thus, it would have been very desirable to provide poly(ethylene terephthalate) filaments with a combination of good thermal stability and good dyeing properties. Such a combination has not been commercially available heretofore.
A large amount of polyester yarn is subjected to a texturing process to increase its bulk. False-twist texturing has generally been the preferred process. Texturability of a polyester yarn is an important requirement, therefore, in the sense that it is required that the polyester yarn be texturable on a commercial false-twist texturing machine without producing a large number of yarn defects, such as broken filaments, or lack of dye uniformity, which may become manifest only in the final fabric.
For many years polyester filaments were melt spun and wound onto a package without drawing at speeds of up to about 1000 meters/minute, e.g. as described in Chapter 5 of Ludewig. This process (which can now be termed "low speed spinning") provided filaments of relatively low orientation (as measured by a low birefringence), relatively low tenacity, low yield-point and relatively high break-elongation. These filaments were not useful as textile yarns until they had been subjected to a drawing process. Thus it was originally standard procedure first to make a package of spun polyester filament and then to subject the filament to a drawing and annealing process which increased tenacity, yield point, orientation (birefringence) and crystallinity, and reduced break-elongation, thus producing "hard" filaments which could be used commercially.
This proceduce was referred to as the "split process" and was expensive, primarily because of the need to operate the stages of the process at different speeds, and therefore, to wind up filaments at each intermediate stage. It has long been desirable to produce hard filaments continuously, i.e. to reduce the number of separate stages involved in hard filament production and thus avoid the need for winding up after any intermediate processes.
For instance, the processes of melt spinning and drawing have been combined into a coupled spin-drawing process without intermediate windup, e.g. as disclosed in Example IV of Chantry & Molini U.S. Pat. No. 3,216,187, wherein the polyethylene terephthalate was melt spun at a (low) withdrawal speed of 500 yards/minute (450 meters/minute), and drawn immediately (i.e. without intermediate windup) 6.times. and annealed before windup at 3000 yards/minute (2700 meters/minute). Coupled spin-drawing produces a drawn yarn of high tenacity, crystallinity, orientation, yield point and reduced break-elongation, i.e. a hard yarn, comparable to drawn yarn produced by low speed spinning and drawing in separate process-stages, i.e. the split process.
In recent years, polyester filaments have been manufactured by a process of "high speed spinning". This typically involves the use of windups operating at speeds, e.g., of 3000 to 4000 meters/minute, similar to those in the coupled spin-drawing process, but is a one-step process in which the polyester filaments are spun and wound directly at a high withdrawl speed, without any drawing step. High speed spinning has been used to produce partially oriented yarns that are particularly useful for draw-texturing, as disclosed by Petrille in U.S. Pat. No. 3,771,307, and this process is now operated commercially on a large scale. The partially oriented yarn that has been produced by high speed spinning has higher orientation (birefringence) and tenacity, with reduced break-elongation, compared to undrawn yarn produced by low speed spinning. The partially oriented yarn produced by high speed spinning has a lower crystallinity than drawn yarn produced theretofore by either a coupled or a split process. Although high speed spinning of polyester filaments had been patented in July 1952 by Hebeler in U.S. Pat. No. 2,604,689, and received further technical attention, e.g. in Section 5.4.1 in Ludewig, and by Griehl in U.S. Pat. No. 3,053,611, it has only been within the present decade that high speed spinning has been commercially practiced.
Hebeler also described, in U.S. Pat. No. 2,604,667, using still higher withdrawal speeds, in excess of 5200 yards/minute (4700 meters/minute), to produce polyester filaments having tenacities of at lest 3 grams/denier and boil-off shrinkages of about 4% or less in the as-spun state. Although this disclosure has been available for more than 20 years, and has been extensively investigated by experts such as Ludewig, it has not been been suggested by such experts that the need for poly (ethylene terephthalate) filaments having the aforesaid combination of properties (enhanced dyeability accompanied by thermal stability over a large temperature range) could have been satisfied by spinning the filaments at extremely high withdrawal speeds.